How to mirror/copy changes to other side of mesh
There are lots of fancy options, but perhaps the simplest and least likely to cause unwanted issues, that will serve you well as a basic tool long into the future, is to just duplicate the vertices and scale by -1.

Please note, you can also just use the mirror tool (see link below), but this method does not depend on having other vertices selected, so it is more versatile.


Delete the vertices on the bad side.
Set the 3D cursor to the middle of the head.
select the good vertices (the mirror of the ones you deleted) (ctrl + shift + M).
shift + D to duplicate.
Set pivot to 3D cursor
s, x, -1 to flip over the X axis.
alt + m to merge the doubles.


Some of the other options are:


Delete half and use mirror modifier.
Symmetrize in edit mode.
Mesh -> mirror ctrl + m in edit mode.